Spring 1967: a Chaparral 2D, Lancer body, with a Classic 26D and a scratch-built brass tube chassis copying Mike Morrissey's article in the April 66 Car Model..... Lots of cold joints, misaligned tubes, etc. Joints probably popped the first time I had it on the track, and never got it to work right... Tore it down to reuse bracket, bearings, motor, etc. I think I learned to solder when I discovered heat transfer and flux!
This doesn't count the bastardized versions of Strombecker and similar chassis that I never seemed able to really finish.
My first successful scratch-built was a pan-chassis (with brass strips, remember those? a very short trend!) under a Lotus 30 with a rewound 16D (60 x 30ga, balanced by EBCO I think, Versitec magnets glued in).